Describe the bug

I am trying to pre-bundle dependencies for a legacy browser target (Chrome 81) by setting optimizeDeps.rolldownOptions.transform.target. What I expect is that Rolldown transpiles dep output to the configured target so that modern syntax like ??=, ?., class fields, etc. gets lowered. What actually happens is that the pre-bundled files in node_modules/.vite/deps/ still contain untranspiled modern syntax — the user-supplied target is ignored.

Root cause

In packages/vite/src/node/optimizer/index.ts (v8, prepareRolldownOptimizerRun), the options passed to rolldown() are built as:

transform: {
  ...rolldownOptions.transform,                     // user config spread first
  target: ESBUILD_BASELINE_WIDELY_AVAILABLE_TARGET, // hardcoded — overwrites user's target
  define
},

In v7, the hardcoded default is placed before ...esbuildOptions:

target: ESBUILD_BASELINE_WIDELY_AVAILABLE_TARGET,
...
...esbuildOptions,                // user config spread last, can override
supported: {
  ...defaultEsbuildSupported,
  ...esbuildOptions.supported,
},

So in v7 optimizeDeps.esbuildOptions.target correctly overrode the default. The v8 rewrite around rolldown flipped the spread order and silently broke this contract.

Suggested fix

transform: {
  target: ESBUILD_BASELINE_WIDELY_AVAILABLE_TARGET,
  ...rolldownOptions.transform,
  define
},

Reproduction

https://stackblitz.com/edit/vitejs-vite-du3yfmdq?file=vite.config.js

Steps to reproduce

1. Vite 8 project with any dep shipping ES2021+ syntax (e.g. vue-router@5 uses ??=).
2. vite.config.js:

import { defineConfig } from 'vite'

export default defineConfig({
  optimizeDeps: {
    rolldownOptions: {
      transform: { target: 'chrome81' },
    },
  },
})

3. Run vite --force to rebuild the dep cache.
4. Open node_modules/.vite/deps/vue-router.js. Expected: ??= etc. lowered for chrome81. Actual: untransformed.

System Info

none

Used Package Manager

pnpm

Logs

No response

Validations

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